I thought I'd reply to this old thread to document my solution to "ML320 Rattle Noise".

I have a 2005 ML350, made a bad metallic rattle noise when put in gear; D or R. Read through many many threads, and pushed me to the conclusion that I must have a bad cat converter.

Upon inspection, I found that both O2 sensors had backed out of their threads and the sensors and the washers were rattling. A pleasant surprise. The vehicle has 140K miles on it, and the sensors looked original, not sure if that's possible. I bought it with 95K.

Anyway, any future rattlers may benefit from this info.